1. Field of the Invention
The present invention relates to a power amplifying apparatus and a radio communications apparatus using this power amplifying apparatus. More particularly, the present invention relates to a power amplifying apparatus having a distortion compensation function and a radio communications apparatus using such a power amplifying apparatus.
2. Description of the Related Art
In radio communications apparatuses such as cellular phones which adopt linear modulation systems including CDMA (Code Division Multiple Access) and the like, power amplifying apparatuses with low distortion and high efficiency are required. As a power amplifying apparatus of such sort, a power amplifying apparatus having a distortion compensation function is known. An example of the configuration thereof is shown in FIG. 13. This conventional power amplifying apparatus shown in FIG. 13 includes an amplification FET 101, an input matching circuit 102, a feedforward path 103, a low-frequency choke inductor 104 and an output matching circuit 105, and suppresses third-harmonic distortion by means of linearization using feedforwarding.
The operation principles for the power amplifying apparatus having the configuration described above are described below. First, by having two sine wave signals or modulated signals inputted via the input matching circuit 102, the amplification FET101 handles large signals. In other words, it is in a state where it is generating high-order harmonics besides a basic wave. Of the second-harmonic distortion waves generated by the amplification FET 101, the low-frequency components are converted into voltage signals by a load resistor 113 after passing through an RF choke inductor 111 and a capacitor 112 of the feedforward path 103. The low-frequency components are amplified by an AF amplifier 114.
The low-frequency components amplified by the AF amplifier 114 are reinjected into a drain of the amplification FET 101 after passing through a capacitor 116 and a choke inductor 117. A portion of the reinjected low-frequency components is converted into third-harmonic distortion due to the non-linearity of the amplification FET 101. Then, at the drain of the amplification FET 101, the third-harmonic distortion that originally existed is cancelled by the newly generated third-harmonic distortion, and third-harmonic distortion components are attenuated as a result.
The principles of the distortion reduction in the feedforward system may be explained analytically with equations as follows.
Input/output characteristics of a power amplifier are shown with a polynomial expression below.Vo(t)=G1·Vi(t)+G2·Vi2(t)+G3·Vi3(t)+Λ  (1)
If the angular frequencies of the two sine wave signals to be inputted to the power amplifier are taken to be ω1 and ω2, the angular frequency of a second-harmonic distortion component in the power amplifier output is ω2·ω1. This component is reinjected into the power amplifier by any suitable method. In the conventional example above, it is reinjected from the drain side of the amplification FET 101.Vi(t)=A cos ω1t+B cos ω2t+H cos {(ω2−ω1)t+φ}  (2)
In equation (2), the second term represents low-frequency second-harmonic distortion components. However, H is a coefficient of the amplitude, and φ is the phase shift amount.
Next, equation (2) is substituted into equation (1). In so doing, instead of listing every term, which makes the equation complicated, only the second-order coefficient terms are listed, the outcome of which is as follows.                                                         G              2                        ·            V                    ⁢                                           ⁢                                    i              2                        ⁡                          (              t              )                                      =                ⁢                                            G              2                        ⁢                                                            A                  2                                +                                  B                  2                                +                                  H                  2                                            2                                +                                    G              2                        ⁢            B            ⁢                                                   ⁢            H            ⁢                                                   ⁢                          cos              ⁡                              (                                                      ω                    ⁢                                                                                   ⁢                    1                    ⁢                    t                                    -                  ϕ                                )                                              +                                    G              2                        ⁢            A            ⁢                                                   ⁢            H            ⁢                                                   ⁢                          cos              ⁡                              (                                                      ω                    ⁢                                                                                   ⁢                    2                                    +                  ϕ                                )                                              +                                    G              2                        ⁢                          {                                                                                          A                      2                                        2                                    ⁢                                      cos                    ⁡                                          (                                              2                        ⁢                        ω                        ⁢                                                                                                   ⁢                        1                        ⁢                        t                                            )                                                                      +                                                                            B                      2                                        2                                    ⁢                                      cos                    ⁡                                          (                                              2                        ⁢                        ω                        ⁢                                                                                                   ⁢                        2                        ⁢                        t                                            )                                                                                  }                                +                                    G              2                        ⁢            A            ⁢                                                   ⁢            B            ⁢                          {                                                           ⁢                                                                    cos                    ⁡                                          (                                                                        ω                          ⁢                                                                                                           ⁢                          1                                                +                                                  ω                          ⁢                                                                                                           ⁢                          2                                                                    )                                                        ⁢                  t                                +                                ⁢                                  cos                  ⁡                                      (                                                                  ω                        ⁢                                                                                                   ⁢                        2                                            -                                              ω                        ⁢                                                                                                   ⁢                        1                                                              )                                                              }                                +                                    G              2                        ⁢            A            ⁢                                                   ⁢            H            ⁢                                                   ⁢            cos            ⁢                          {                                                                    (                                                                  2                        ⁢                                                  ω                          1                                                                    -                                              ω                        2                                                              )                                    ⁢                  t                                -                ϕ                            }                                +                                    G              2                        ⁢            B            ⁢                                                   ⁢            H            ⁢                          {                                                                    (                                                                  2                        ⁢                                                  ω                          2                                                                    -                                              ω                        1                                                              )                                    ⁢                  t                                +                ϕ                            }                                                          (        3        )            
Since the last two terms in equation (3) have angular frequency components 2ω1·ω2 and 2ω2·ω1, they are third-harmonic distortion components (IM3). If these terms can cancel the third-harmonic distortion components generated in the power amplifier, it would be possible to attenuate third-harmonic distortion components.